1. Field of the Invention
This invention relates generally to a method and apparatus for bonding polytetrafluoroethylene (PTFE) to a metal substrate. An intermediary layer, preferably fluorinated ethylene propylene (FEP), is completely melted while the PTFE is partially melted to form a strong mechanical bond between the PTFE and the metal substrate.
2. Description of Prior Developments
Prior attempts to bond PTFE to metal have met with varying degrees of success. For example, U.S. Pat. No. 3,462,333 to McCormick discloses a method of bonding a PTFE wafer to a metal casing using an intermediary bonding layer of FEP. Although a bond may be formed using this method, the physical properties of the PTFE are altered so as to render the PTFE unsuitable for certain applications. More particularly, the entire PTFE element in McCormick is subjected to high temperatures which are sustained over a period of time sufficient to melt and resinter the entire PTFE element and thereby alter its internal physical structure, particularly its crystallinity.
When PTFE is heated to its gel or melting temperature of approximately 621.degree. F., its crystallinity begins to decrease. When this occurs, the physical properties of the PTFE begin to change. The lower the crystallinity of a PTFE element, the greater is its elastic memory. With increased elastic memory, a sintered PTFE element will return to its unstressed form more quickly and with greater force. Thus, as the crystallinity of the PTFE decreases, so does its dimensional stability as the PTFE gradually and unpredictably returns to its unstressed condition. This poses a significant problem in sealing applications where a PTFE sealing element must be accurately dimensioned to form a secure seal against a coacting member such as a shaft, housing, bore or the like.
Another problem associated with prolonged or excessive heating of a PTFE element is the loss of certain physical properties which may have been optimized during its initial production. That is, not only may cyrstallinity be optimized during the initial sintering of a PTFE element, but so may wear resistance. If a sintered PTFE element is held at temperatures at or near its gel temperature for any period of time, it is resintered and begins to experience molecular degradation wherein long molecules of PTFE are broken to form shorter molecules. Ideally, PTFE should be sintered only once in order to preserve its strength and wear resistance.
The shorter molecules formed during molecular degradation increase the crystallinity of the PTFE element. This results in a loss of resilience as well as a loss in wear resistance. Molecular degradation begins as soon as PTFE reaches its gel temperature. The greater the temperature of the PTFE above its gel temperature, the quicker is the rate and the greater is the degree of molecular degradation.
Thermal expansion poses another problem when heating and melting PTFE. Thermal expansion can increase the volume of a PTFE element up to about 20%. If a PTFE element is not constrained during heating, it will expand in a generally unpredictable manner and upon cooling and contraction will typically lose any carefully controlled dimensions formed previously. This is particularly troublesome if the PTFE element is used as a sealing lip which must maintain accurately controlled contact with a coacting sealing surface.
If the PTFE element is constrained in a mold or the like during heating, internal stresses will develop within the element. These stresses can, over time, produce dimensional changes in the element resulting in a corresponding loss of precision tolerances. On the other hand, if the PTFE element is not constrained during heating, all precision dimensions will likely be lost immediately. For example, a previously sintered PTFE element which has been machined to tolerances of +0.005 inch can easily deform during heating so as to produce post-heated tolerances of +0.020 inch.
A sintered PTFE element is, in effect, stress relieved during further heating and sintering and returns to a form which approximates its stress-free configuration. In the case of a PTFE element which has been precision machined for use as a radial lip sealing member, the loss of precision dimensions upon heating can result in unpredictable loading and wear between the PTFE sealing member and a rotating shaft or the like. Moreover, the contact pattern between the sealing member and shaft will likewise become unpredictable. In each case, poor sealing performance can be expected in the form of seal leakage or premature seal wear and premature seal failure.
Prior attempts to melt and bond PTFE to a metal substrate under heat and pressure have resulted in a dilemma that has heretofore remained unresolved. That is, the use of high melting temperatures for short periods of time has resulted in molecular degradation, while the use of lower melting temperatures (at or slightly above the gel temperature) for longer periods of time has also resulted in molecular degradation. In each case, by heating a PTFE element to a given temperature for a sufficient period of time to form a satisfactory bond, a loss of desirable physical properties has resulted.
When a PTFE element is heated at or slightly above its gel temperature, it takes a relatively long period of time for the entire element to gel. This is due to the low thermal conductivity of PTFE which reduces or slows the rate of heat transfer therethrough. If one attempts to avoid molecular degradation of a PTFE element by heating the element at or near its gel temperature, (as opposed to higher temperatures) it will take so long to completely melt or gel the entire element that the PTFE material which is initially gelled will remain gelled for so long that it will experience molecular degradation by the time the last of the PTFE material reaches its gel state. However, if one attempts to increase the rate of heat transfer by using higher melting temperatures and thus reduce the time the PTFE is in its gel state, the rate of molecular degradation has heretofore increased to unacceptable levels thereby causing a significant loss of physical properties.
Accordingly, a need exists for a method and apparatus for bonding PTFE to a metal substrate while minimizing molecular degradation and maximizing the dimensional stability of a PTFE element . A need also exists for a method and apparatus which reduces PTFE bonding time and increases production efficiency.